Testing: Jest, pytest, Unit Tests, Integration Tests, TDD

🎯 Learning Objectives

• Write unit tests with pytest for Flask APIs
• Write Jest tests for React components
• Implement integration tests for full-stack features
• Apply test-driven development (TDD) principles

📖 Core Content

1.1 pytest for Flask Backend

python
Run Inline
# runnable
# test_api.py
import pytest
from app import app
@pytest.fixture
def client():
    app.config['TESTING'] = True
    with app.test_client() as client:
        yield client
def test_login_success(client):
    """Test login with valid credentials."""
    response = client.post('/auth/login', json={
        'email': '[email protected]',
        'password': 'correct_password'
    })
    assert response.status_code == 200
    assert 'token' in response.json
def test_login_invalid_email(client):
    """Test login with invalid email format."""
    response = client.post('/auth/login', json={
        'email': 'not-an-email',
        'password': 'password'
    })
    assert response.status_code == 400

1.2 Jest for React

javascript
Run Inline
// runnable
// LoginForm.test.jsx
import { render, screen, fireEvent } from '@testing-library/react';
import LoginForm from './LoginForm';
test('renders login form with submit button', () => {
  render(<LoginForm />);
  expect(screen.getByText('Login')).toBeInTheDocument();
  expect(screen.getByLabelText('Email')).toBeInTheDocument();
});
test('shows error for invalid email', () => {
  render(<LoginForm />);
  fireEvent.change(screen.getByLabelText('Email'), {
    target: { value: 'invalid' }
  });
  fireEvent.click(screen.getByText('Login'));
  expect(screen.getByText('Invalid email format')).toBeInTheDocument();
});

1.3 Why This Matters

2. 📝 Practice Questions

Q1: A bug report says "User registration fails silently." After fixing, write a test to prevent regression.

python
Run Inline
def test_registration_validation(client):
    """Registration with missing fields should return 400."""
    # Missing last_name
    response = client.post('/auth/register', json={
        'first_name': 'John',
        'email': '[email protected]',
        'password': 'Password123'
    })
    assert response.status_code == 400
    assert 'last_name' in response.json['errors']

This test ensures: (1) the API validates all required fields, (2) it returns a clear error message, (3) future changes don't accidentally remove this validation. Join Discord PreviousFile Uploads & Background TasksNextPerformance Optimization